Semiconductor lasers tend to have output spectra consisting of multiple longitudinal modes. Depending on the relative strengths of these modes, the lasers are classified as multi-mode or single mode. Single mode lasers are nominally single mode because their output comprises a dominant main mode and several small, yet measurable, modes at wavelengths near the wavelength of the main mode. While the average power in each side mode is usually quite small, the side modes are generally present. Moreover, the power of each individual longitudinal mode fluctuates from a zero power level to significant percentage of a full output power level at any time.
When a single mode laser is directly modulated to carry information in a lightwave communication system, the spectrum of each modulated digit comprises light output from the laser at the wavelengths corresponding to the main mode and to those side modes which were present during the formation of the particular information digit. After the modulated digit traverses a length of dispersive optical fiber, the different wavelengths of the longitudinal modes cause the modes comprising a digit to lose temporal correlation which may result in an error at the receiver. The error phenomenon caused by mode partitioning of the transmitting laser is especially exhibited in ASK and OOK systems. For systems employing high speed modulation and with transmission over long distances, mode partitioning degrades error rate performance and often introduces so-called "error rate floor" characteristics in the error rate versus average optical power system specifications. While lasers are being constantly refined to eliminate or just ameliorate the effects of mode partitioning, it is necessary to determine the propensity of the laser toward mode partitioning and degree of mode partitioning for each laser prior to installation in a system.
Various techniques have been demonstrated for capturing mode partitioning data from a modulated laser so that the performance of the laser may be characterized. The variety of techniques cover eye diagram mapping of the error rate, side mode suppression ratio analysis, inducing error by varying total dispersion, analyzing kinks in light-output versus current (L/I) curves, and mode sampling by spectrometer. In general, the latter technique is most widely accepted.
In eye diagram mapping, the receiver decision point is varied in amplitude and phase to collect an ensemble of different error rates. The laser under test transmits its signal over a full transmission system to the lightwave receiver. By plotting constant error rate contours as closed loops, it is possible to understand quantitatively the error rate distribution in the eye pattern. See, for example, J. of Lightwave Tech., Vol. 6, No. 5, pp. 678-685 (1988). It is said that this technique and the eye pattern supply information about degraded performance resulting from an error rate floor and low probability phenomena which characterize laser performance. Even for high data rate systems, this test procedure requires long periods of time for data collection to obtain a statistically significant sampling of laser performance. Clearly, this technique is time consuming and provides only circumstantial evidence of mode partitioning in characterizing the laser at the remote end of the communication system. The latter detriment is so because the eye pattern data is assembled by collecting only error rate data from the receiver and then by stating without further supporting data that error performance of the laser was caused by mode partitioning events rather than by collecting mode partition event data together with the error rate data from the laser under test and then trying to correlate that partition event data with the error rate data.
Side mode suppression ratio analysis and analysis of kinks in the L/I curve for the laser have been found to provide a rough approximation of mode partitioning activity. For a description of the former technique, see J. of Lightwave Tech., Vol. 6, No. 5, pp. 636-642 (1988). While these techniques are known to be useful, their utility is realistically limited to obtaining merely indications of mode partitioning without qualitative support.
For the mode distribution sampling by spectrometer technique, a number of references have shown similar experimental configurations. See, for example, IEEE Trans. on Commun., Vol. COM-28, No. 2, pp. 238-243 (1980); J. of Lightwave Tech. Vol. LT-2, No. 1, pp. 44-48 (1984); and J. of Lightwave Tech. Vol. LT-3, No. 3, pp. 706-712 (1985). In general, these references show the laser output focused on a monochromator which is resolved to pass one longitudinal mode of the laser at any given time. High speed sampling is then used to record a historical record of the fluctuations of the selected mode being passed by the monochromator. Since many other longitudinal modes are rejected by the monochromator and after a statistically significant amount of data has been collected for the one mode being analyzed, it becomes important for completeness of this technique to adjust the monochromator to pass another longitudinal mode of the laser. Unfortunately, this technique is extremely time consuming, while producing mode partition data which is inaccurate because the data about partition events are collected sequentially from one longitudinal mode to the next. Infrequent, yet very important, partition events in other modes are missed when the monochromator is not resolved on the modes which are experiencing a partition event. As a result, statistical techniques are used during later data analysis to try to fill in missing data about partition events. It is also noted that mode sampling by spectrometer techniques are both polarization sensitive (dependent) and inherently lossy. Polarization sensitivity introduces additional complexity in the test procedure by requiring that the laser and the test equipment be aligned to optimize mode coupling and power transfer. Loss, on the other hand, is problematic when one considers the usual low power emitted in the modes to either side of the main (central) longitudinal mode of the laser.